This invention is an aqueous coating composition that is well suited for coating metal surfaces and exhibiting water resistance at elevated temperatures. Thus, these compositions are particularly adapted to coating cans that are subjected to high temperature processes during or after filling.
The surfaces of metallic containers intended to contain food or beverages are typically coated so as to retard corrosion of the container and to improve the appearance of the surface. Food and beverage cans are generally made from aluminum, steel, blackplate or tinplate steel. These metal containers are subject to air oxidation and to corrosive action of the food or beverage products, both of which can be accelerated during high temperature pasteurization or sterilization processes associated with some canning operations.
In order to provide high temperature process resistant coatings on cans, the prior art has generally relied on coating compositions that contain substantial amounts of organic solvent. Volatilization of these organic solvents during the curing of these coatings is considered environmentally undesirable, and therefore the art has considered it desirable to find water based coatings with the required degree of high temperature process resistance. Water based coating compositions are generally based on polymeric binders that are either water soluble or dispersions in water. Water dispersions usually entail synthesis by emulsion polymerization using a surfactant. The presence of the surfactant in the film formed from such a coating composition degrades the water resistance of the film. An example of this type of water dispersion composition is disclosed in U.S. Pat. No. 4,305,859 (McEwan et al.). Water soluble binder systems also generally suffer from poor water resistance because of the relatively high carboxy group content needed for water solubility. Such conventional water based coatings typically soften, blister, or blush (get hazy due to water absorption) under process conditions, e.g., 250.degree. F. (121.degree. C.) steam for 30 minutes). The prior art has attempted to improve water resistance of these types of compositions by including substantial amounts of aqueous amino resins such as alkoxy derivatives of melamine or benzoguanamine, but a disadvantage to that approach is typically a loss of film flexibility and the undesirable generation of formaldehyde as a product of the curing reaction. Additionally, the cross-linking stability provided by these amino resins is less than desired under the conditions of high temperature water exposure.
A water based can coating that has high temperature process resistance without relying primarily on melamine or benzoguanamine derivatives or the like would be highly desirable. A high level of process resistance would be indicated by the ability to withstand 250.degree. F. (121.degree. C.) steam for 30 minutes. Superior process resistance would be even more desirable, as evidenced by the ability to withstand 250.degree. F. (121.degree. C.) steam for 90 minutes.
U.S. Pat. No. 4,076,917 (Swift et al.) discloses that hydroxyalkylamides can be used to react with carboxy groups of a waterborne acrylic polymer so as to cure the polymer. The patent discloses that in order to effect a cure, the amount of hydroxyalkylamide used should be chosen relative to the amount of carboxy groups in the acrylic polymer. More specifically the ratio of hydroxy groups in the hydroxyalkylamide to carboxy groups in the polymer are disclosed to be in the range of about 0.5:1 to about 2:1, preferably 1:1. In all of the examples of that patent in which the ratio is disclosed it is 1:1 (or "stoichiometric").